You definately want to spend the time to encode all your clips to the same codec. I personally use Indeo5.10 - this has given me good results, and increased stability. A dedicated installation of Windows is great - no virus protection, firewall, etc - only codecs, drivers, 7 resolume. Make sure you have all the latest directx drivers, video card drivers, and onboard agp controllers.
Having fast hard drives, configured in a stripe 0 Raid array, with the clips evenly distributed among them. Spin speed is key. I hear people say it's all about gigahurtz, but I'm running on an Athlon64 3000+, 1gb 400Mhz ddr, radeon 9800 pro 256, 3x 7200 8mb buffer hd in Raid 0 stripe. Hauppauge WinTv capture card, ATI TV Wonder Pro, USB 2.0 webcam.
The Athlon64, despite it not truly processing in 64 bit, really seems to perform for me, I'm curious what a P4 3.0 + would be like.
Read some threads on RAM management. If you can find a stable ram drive program, try using it to load clips.
As far as performance, avoid inputs that require processor use. DV or firewire inputs eat up cpu and slow down resolume. I find tv cards work best because they have their own processors. Even the USB camera I use slows down resolume a little when loaded in a layer.
Keep your computer as cool as possible. Heat will slow you down, and possibly shut you down. I've got 3 120mm fans around my hard drives, which now sit on heat syncs. 12 fans total in case.
I'm processing at 640x480 and outputting at 800x600 all in 32 bit.
I am pleased for the most part with the results.